In an article entitled "Resistivity Testing for Quality Control," appearing on pages 1066-1074 of the August 1986 issue of the publication Materials Evaluation, (the "Materials Evaluation" article) and incorporated herein by reference and made a part hereof, there is disclosed apparatus adapted by testing for resistivity conditions in machine parts to provide indications of defects or other features of such parts. That apparatus comprises a probe mounting head, a pair of outer current probes mounted by and projecting forward from said head and spaced apart by a gap, a pair of similarly mounted and projecting inner voltage probes disposed in said gap between said current probes, a current source connected to said current probes for supplying them with current of predetermined value, and indicating means connected to such voltage probes for sensing, measuring and indicating voltages developed between them.
In operating the apparatus, the head is brought to a mechanical part to be tested to produce contact between that part and all four probes. Current of predetermined character and value is then caused to flow through the part between the current probes in contact therewith to produce between the voltage probes a voltage of a value which, in accordance with the classic equation in electricity, V=IR, varies directly with such current as multipled by a quantity .rho..sub.a denoted the apparent resistivity of the region of the part explored by the current flow therethrough. It follows that if the effective value of the current can be treated as a constant, the voltage measured between the voltage probes will be in direct proportion with the value of .rho..sub.a.
The value of .rho..sub.a is, in itself, however, a function of at least three parameters as they exist in such region, namely the composition of the material of the part and the resulting bulk resistivity of such material, the configuration of the part (e.g., its thickness, nearness of edges to the probes, etc.) and the degree of presence of structural defects (e.g., cracks, voids, dislocations) in the part. Assuming however that, in testing a series of parts, the first two factors of composition and configuration can be relied upon as not varying enough from part to part to produce effects in the total value of the measured voltage which would tend to mask changes caused in such value by a defect or defect sought to be detected, then the apparatus can be used to screen out all parts in such series which have such defects to an unacceptable degree. That is, the apparatus can be calibrated by applying the probes to a part known to be acceptably free of defects, noting the value then obtained of the voltage produced between the two voltage probes, and utilizing such value as a standard value with which are compared the voltage values obtained from probing the other parts. The ones of such parts yielding measured voltages which deviate little enough from the standard value to fall within a predetermined range of acceptability are parts which will be kept. Parts which do not meet that criteria will be rejected.
Instead of using the four-probe technique to screen part(s) or all of machine parts, structural assemblages or the like for structural defects, such technique may be employed to provide a measure of the degree of presence in such items of some other feature of interest. For example, the depth of case hardening of drive shafts will, by virtue of changing the composition and, hence, bulk resistivity of the shaft material through which current flows in using such technique, produce commensurate changes in the total voltage generated between the voltage probes by such flow in the course of such use. Hence, if it can reasonably be assumed that the effects on such total voltage of variations therein caused by variations in the configuration of or by defects in the drive shafts being inspected are effects which can be ignored as insignificant, then such variations in such total voltage can be utilized to provide a useful measure of such depth of case hardening.
Such four-probe technique for evaluating electroconductive bodies has, however, the problem that such masking effects can occur with undesirable frequency in the practical application of such technique. Another problem which can arise in such application is that, even though the contributions to the total measured voltage derived from variations in the factors affecting such voltage but irrelevant to the feature sought to be measured can be small enough so as to not mask out the contribution to such voltage caused by the presence or absence of, or variations in, such feature, nevertheless, such contributions from such irrelevant parameters can be large enough as to make the selection of a particular range of deviation from such standard value of voltages obtained by such technique from inspected parts an undesirably imprecise "yardstick" of which of such parts really ought to be accepted and which should not.